My husband and I did 12 DCL trips without kids. (We only recently had additions to our family!) The thing that you have to remember is that Disney is PHENOMENAL at entertaining ALL ages. If you enjoy going to the parks as adults, you'll enjoy the ships as adults tenfold! Just don't go expecting a typical "party-ship" environment. No casino, no massive amounts of booze and drunken Guests, no wildly inappropriate stage acts. (All of this is a GOOD thing, if you ask me!) Granted, in the clubs at night, the adult-only cabaret style shows can get a touch risqué, but all entertainers seem to know where the line is onboard DCL.
One really nice change that they made to the newer ships is that the clubs are actually a little smaller and more compact. Seems like it would be a negative, but it's not. See, I don't know about all of you, but one thing I always disliked about Wavebands and Rockin' Bar D is that when they aren't super busy, (typically any time after 11pm), the dance floor dwindles down to just a few people. Because the clubs are so large, it makes them seem totally empty and dull. The clubs on the Fantasy and Dream tend to look pretty full, even with only a few parties inside, so they are more welcoming and exciting.
Also, since the Dream and Fantasy were built after Disney was able to look at what people liked and disliked about the Magic and Wonder, the newer ships have a lot of great enhancements for the adult only areas. The spas are gorgeous, there's more tucked away, private spots to lounge, both indoors and out, and the club areas are great! Not to mention, Remy, which is not available on the older ships.
Don't get me wrong..... I LOVE the Magic and Wonder. In fact, I may even go so far as to say that The Magic is my favorite ship! But I would say that for a couple, don't hesitate to book the Fantasy! She's got plenty to offer! ENJOY!